This invention relates to method of preparing metal chromite dehydrogenation catalysts.
Recently it has been found that magnesium chromites are excellent dehydrogenation catalysts (U.S. Pat. No. 3,781,376). Prior to this discovery, chromia-alumina catalysts had been considered far superior to other materials for dehydrogenations. The dehydrogenation process is a cyclic process in which gaseous hydrocarbons such as butane, isobutane, isopentane or ethylbenzene are dehydrogenated over a suitable catalyst to produce butenes and butadiene, isopentene and isoprene and styrene, respectively. After each dehydrogenation cycle there is a catalyst regeneration cycle in which the accumulated coke is burned off by passing molecular oxygen through the catalyst followed by another dehydrogenation cycle and so on.
The chromia-alumina catalysts are prepared by treating activated alumina with a solution of chromic acid, draining off the excess acid from the alumina, drying and heat treating at about 1400.degree.F. Commercial chromia-alumina dehydrogenation catalysts normally contain about 20% chromium oxide. Preparative methods are shown, for example, in U.S. Pat. Nos. 2,399,678 and 2,419,997.
Metal chromite formation can be accomplished by reacting an active compound of chromium with an active compound of magnesium and the other designated metals. By active compound is meant a compound which is reactive under the conditions to form the chromite. Starting compounds of chromium, magnesium or the other metals may be such as the nitrates, hydroxides, hydrates, oxalates, carbonates, acetates, formates, halides, oxides, etc.
The magnesium chromite can be prepared by precipitation, dry or wet milling or mixing, by precipitation of one of the ingredients in the presence of the other, coprecipitation and impregnation of one or more of the solid ingredients with aqueous or non-aqueous solutions of salts of the ingredients. The coprecipitation of the chromite precursors is a preferred method of obtaining intimate mixing thereof; however, prior methods of obtaining coprecipitate have had serious drawbacks which had been contraindicative of their use.
The prior method of coprecipitation consisted of dissolving soluble compounds of a metal, such as magnesium, and chromium in an aqueous solution and precipitating an insoluble mixture of hydroxides by using ammonium hydroxide and sodium or potassium hydroxide. Generally, Na and K hydroxides were not employed since residual alkali metal tends to deactivate the resultant chromite. Ammonium hydroxide does not have this defect and was usually employed for obtaining coprecipitates of magnesium and chromium compounds in the past; however, the ammonium salts in the supernatant liquid tended to resolublize the precipitates.
It is a feature of the present invention that a method of obtaining coprecipitation of metal and chromium components of the chromite has been devised which does not require hydroxide precipitates. It is a particular feature of the present invention that a precipitate is recovered which produces a high-quality magnesium chromite dehydrogenation catalyst. An advantage of the present invention is a very simple procedure which is easily driven to completion and the products produced therein do not require extensive purification to employ them to prepare the chromite dehydrogenation catalyst. These and other features and advantages of the present invention will be clearer from the following discussion and description.